This invention relates to ammunition for firearms, and in particular to tubular projectiles for small arms such as rifles and shotguns.
Tubular projectiles have been known for some time and have been found to provide significant advantages over conventional ammunition in certain applications. Conventional ammunition typically comprise a solid mass with a rounded nose or ogive portion, a generally cylindrical body, and an aft or tail portion terminating abruptly in a flat surface normal to the longitudinal centre axis of the cylindrical body.
The aerodynamics (ballistics) of solid projectiles such as conventional ammunition is fairly well understood. The relatively blunt nose produces a very high drag force and a parabolic shock wave when the projectile is fired at high velocity. The blunt tail section produce considerable turbulence behind the projectile which translates into further drag from conversion of energy from the projectile to the surrounding mass of air.
While the aerodynamics of tubular projectiles is generally less well understood than those of solid projectiles, the hollow centre passage in tubular projectiles has been found to address some of the problems with conventional ammunition. In particular, tubular projectiles have been found to have reduced total drag due to the hollow centre passage and thereby reduced frontal area, which in turn generally leads to better flight characteristics and increased impact velocities.
From a technical ballistics perspective it has been speculated by Flatau in U.S. Pat. No. 4,301,736 that the normal bow shock wave found in solid ammunition is not present under ideal supersonic flow conditions in tubular projectiles, resulting in a dramatic reduction in total drag force. This flow condition requires certain precise combinations with regards to cross sectional size of the internal and external surfaces of the tubular projectile and the launching or firing velocity.
It has been further proposed that after a tubular projectile is fired and thus begins to decelerate, that the internal air flow can change dramatically whereby a bow shock wave appears at the nose of the projectile and subsonic flow occurs through the centre passage. This condition is called "choking" and is accompanied by a sharp increase in drag. This can result in a tubular projectile beginning to "tumble" in flight causing significant loss of accuracy and range.
To control this phenomenon, it is desired to design a tubular projectile so that the bow wave is "swallowed" and remains thus through a certain range of velocities. This obviously is an important design consideration which must be addressed by those seeking to improve this type of ammunition.
There are other design trade offs or compromises inherent with tubular projectiles. These also have likely limited the range of uses for tubular projectiles to date. Paramount among these are the reduced mass of the projectile and the sometimes less than optimal energy transfer to the target due to the "sharper" leading edge. Most attempts to improve tubular projectiles have focused on minimizing these less than desirable effects, while retaining the inherent advantages with this type of ammunition design.
Tubular projectiles have been known to be used in a variety of ammunition types. These would include conventional primed case ammunition for rifles, and full bore shotgun ammunition.
One type of ammunition that has not been known to utilize tubular projectiles is sub-calibre projectile case-type ammunition for shotguns, sometimes called sabot ammunition.
Solid projectile sabot ammunition has been found particularly popular for deer hunting as many jurisdictions, particularly in the United States, prohibit the use of rifles for deer hunting. Sabots have been found to offer better range and overall performance than most other standard shotgun ammunition.
"Sabot" does not actually refer to the entire ammunition type, but is actually a term referring only to a sleeve, shim or other support to centre a sub-calibre projectile in a gun bore. Often sabots are found in multiple sections and most commonly in sabot segments or sabot halves. As the term is commonly used however, sabot may refer to either the sleeves, shims etc., or the entire ammunition type. Upon firing, the sabot halves are intended to separate from the projectile after the entire assembly leaves the gun muzzle. A number of different sabot systems have been developed, but it has been found that such systems for use in a shotgun, i.e., where it is desired to use a single sub-calibre bullet and a sabot loaded into a standard shotshell, could be substantially improved, particularly in terms of accuracy and flight characteristics.
While the following description may make specific reference to shotguns and/or shotgun shells, it is not intended that the invention be so limited.
Some of the problems encountered in providing a sabot bullet for a shotgun include the fact that while standards do exist, there still exists a large number of older firearms with uncertain and nonstandardized variations in shotgun bore diameters, length, configurations and interior taper or choke. The shellcase diameter will normally exceed the bore diameter or the choke, and therefore any load component, e.g., projectile, wadding, sabot etc., must either be of a lesser diameter than the minimum choke diameter, or be formed of a material which may compress or otherwise be capable of deformable flow to pass through the choke.
Another problem that must be considered is that if a sub-calibre bullet is loaded in a shot shell over a conventional wad column, the inertia of the bullet will cause it to penetrate the wadding when the shell is fired. However, even if a suitable wad material was available, which would avoid penetration due to the bullet inertia, the same inertia or setback forces would deform a projectile made of lead or a lead alloy, thereby necessitating a steel bullet which sacrifices density and ease of fabrication. Attempts to solve this problem have included the use of an "air wedge". This is typically a soft plastic disk which is inserted into the back of the projectile, which seals against the projectile when it is fired.
The setback forces which would deform a projectile are substantial, and if a shotgun projectile is only supported around its circumference with a sabot of desirably light weight and compressible material, the inertial forces have been heretofore considered a difficult problem to solve.
A partial solution to the above problems employing solid projectile sabot slugs is found in U.S. Pat. No. 3,726,231. This patent teaches a solution where the projectile-sabot configuration and relationship is such that about one-half of the face of the wadding is covered by the base of the bullet and the other half covered by the base of the sabot. The greater portion of the force imposed upon the sabot base is transferred to the forward portion of the projectile. Also, with matching the complementary confronting surfaces on the projectile and sabot, all axial forces resulting from setback are distributed evenly. This is generally accomplished by providing the projectile exterior with a medial portion of reduced diameter and then tapering outwardly towards the front and base portions thereof. The exterior surface of the sabot or sabot segments conform to such exterior projectile surface.
When the shell is fired, the inclined surfaces of the projectile and the sabot moving under setback stress, cause the segments of the sabot to spread. This allows the projectile-sabot system to be made with a small enough diameter to be loaded into a shellcase of uncertain interior tolerances. The interior diameter of the shellcase may also be larger than the diameter of the barrel. In addition, the chamber of the gun may be of uncertain length as may the forcing cone. The sabot-projectile, by this expansion of the sabot segments, maintains a snug fit while travelling through these uncertain and varying tolerances. This is a desirable function of and "payload" (projectile, shot, slug, etc.) in a shotgun, otherwise the wadding may not effectively seal the propelling gases.
A further function of setback or inertial forces acting upon the engaged inclined surfaces of the projectile's exterior and the sabot's interior, is the unmistakable tendency of this action toward centring the mass of the projectile in the exact centre of the bore. While previous shotgun projectiles have been designed to compress or "swage down" as they passed through the choke of a shotgun, there has been no design provision to ensure they would do so evenly and keep their mass centred in the bore.
As the sabot-projectile travels down a shotgun bore, a point may be reached where the propellent has been entirely burned or at least is not longer effectively generating propelling gas. At this point, interior bore pressures will drop rapidly and the sabot-projectile will cease accelerating. Since the circumferential surface of the sabot is in contact with the gun bore, the resulting friction will make the sabot tend to travel more slowly than the projectile. In this circumstance, it will encounter a "set forward" instead of a setback of the projectile. Now the inclined surfaces on the rearward portion of the projectile and sabot become active again. Previously, these surfaces were active in keeping the projectile positioned and secured in the loaded shellcase and to keep the projectile from being moved forward in the sabot by surge pressures or the priming charge during the resistance the shot encounters while opening the shotshell crimp and/or entering the forcing cone.
As the projectile moves forward in the sabot, the projectile's sabot is prevented from premature separation. Also, the rear inclined surfaces perform the function of centring the projectile's mass in the bore, and keeping the sabot segments spread into snug, accuracy enhancing bore fit.
In addition to the foregoing, it is essential that the sabot-projectile leave the muzzle as a stable single projectile so as to avoid any tumbling tendency, and the entire assembly is weight stable. Next, after leaving the muzzle, it is necessary that the sabot segments separate from the projectile without imparting an uneven force as they drop away. The sabot segments, after initial opening, can only have contact with the projectile at a point rearward of the projectile's centre of balance. Further, the segments are usually constructed that as they open and begin to fall away, they will continue to turn outward and thus will not disrupt the stabilizing air flow over the projectile.
The projectile itself is constructed to not only cooperate with the sabot segments as above described, but is itself stabilized with its centre of balance or centre of gravity positioned forwardly of its geometric centre. Additionally, the projectile is aerodynamically stabilized, i.e., the least surface is presented to the air in straight forward flight.
The '231 patented slug became the industry standard for some time. Other manufacturers such as Winchester have modified the standard slug somewhat, however still as a solid projectile.
The present invention utilizes some of the concepts in the basic body shape of these prior art devices in its tubular projectile. Significant modifications are made to the leading edge and trailing edges to achieve proper aerodynamics. As well, a unique sabot half system has been developed to assist with proper release.
To date, none of the prior art tubular projectile devices nor the prior art solid projectile devices described or known have been able to achieve all of the performance characteristics of the present invention. Thus, present invention seeks to address the previous limitations and provide a generally improved tubular projectile, with a tubular projectile sabot being one particular application. Again, other types of applications such as full bore shotgun or conventional cased rifle ammunition are possible.